1. Field of the Invention
The present invention relates to an actuator, specially an actuator for a lens driving mechanism, which activates, for example, a pickup in an optical disk drive or a lens in a camera, and more particularly to an actuator for a lens driving mechanism, in which a lead screw rotated by a motor is incorporated.
2. Description of the Related Art
Conventionally, a stepping motor with a lead screw is widely used as an actuator for activating a pickup in an optical disk drive such as a CD drive, a DVD drive or the like, or for activating a lens in an automatic focus control system or a zoom system of a camera. For moving such a lens in the optical axis direction, a gear formed at a portion of a lens frame is adapted to mesh with a threaded portion of the lead screw of the motor (refer to, for example, Japanese Patent Application Laid-Open No. 2000-275497).
FIG. 6 shows a conventional lens driving mechanism disclosed in Japanese Patent Application Laid-Open No. 2000-275497. A lens 30 is held by a lens frame 32, and a guide member 34 is fixedly attached to the bottom portion of the lens frame 32. The guide member 34 has a throughhole 34A, and a guide rod 36 fixed to a lens cylinder (not shown) is passed through the throughhole 34A. Accordingly, the lens 30 is supported slidably in the axis direction of the guide rod 36 (direction of an optical axis L).
On the other hand, a retention member 40 to which one end of a pin 38 is fixed is fixedly attached to the top portion of the lens frame 32, and a rectangular block member 42 made of resin, such as plastic, is attached to the other end of the pin 38. A threaded hole 42A is formed through the center of the block member 42 in the direction of the optical axis L, and a feed screw 44 is engaged with the threaded hole 42A.
Consequently, the lens 30 is supported by the guide rod 36 as well as the feed screw 44 so as to be movable in the direction of the optical axis L. In this connection, since the feed screw 44 works well as a guide rod for the lens 30 due to its engagement with the block member 42, a dedicated guide member for supporting the top portion of the lens 30 does not have to be provided.
The both ends of the feed screw 44 are rotatably supported respectively by front and rear arms 46A and 46B of a frame member 46 having a square U shape. A stepping motor 48 is attached to the rear arm 46B of the frame member 46, and an output shaft 48A of the stepping motor 48 is jointed to the feed screw 44. Consequently, when the stepping motor 48 rotates, the feed screw 44 jointed to the output shaft 48A of the stepping motor 48 rotates, whereby the block member 42 whose threaded hole 42A is engaged with the feed screw 44 is caused to move along the axis of the feed screw 44, in conjunction with which the lens 30 moves linearly in the direction of the optical axis L.
The feed screw 44 includes a threaded portion 44A which has a screw thread (male screw), and plain portions 44B, 44B which have no screw thread and are located respectively at the both ends of the feed screw 44 so as to sandwich the threaded portion 44A. The block member 42, when positioned at the threaded portion 44A, meshes with the feed screw 44 via the threaded hole 42A and is caused to move along the axis of the feed screw 44 with the rotation of the feed screw 44. On the other hand, when positioned at any of the plain portions 44B, 44B, the block member 42 is free from engagement with the feed screw 44 thus becoming freely slidable, which prohibits the block member 42 from moving along the axis of the feed screw 44 even when the feed screw 44 rotates.
In view of the above mechanism for controlling the position of the lens 30, the stepping motor 48 is controlled and driven so that the block member 42 travels within a range where the block member 42 is kept in engagement with the threaded portion 44A. However, if the stepping motor 48 goes out of control and runs abnormally causing the feed screw 44 to keep rotating in the same one direction, the block member 42 moves off from the threaded portion 44A to one of the plain portions 44B, 44B.
Under the condition described above, the block member 42 is disengaged from the feed screw 44, and the feed screw 44 rotates freely alone. Thus, with this mechanism, when the stepping motor 48 runs abnormally, the lead screw 44 is prevented from keeping engagement with the block member 42, which consequently allows the lead screw 44 to rotate idle and therefore prevents damages of the screw threads formed on both the feed screw 44 and the block member 42
Plate springs 50A and 50B urging the block member 42, which is moved to the plain portions 44B and 44B of the feed screw 44, back toward the threaded portion 44A are disposed respectively at the front and rear arms 46A and 46B of the frame member 46. So, in the case if the stepping motor 28 runs abnormally and drives the block member 42 to either of the plain portions 44B, 44B of the feed screw 44, when the stepping motor 28 starts running normally again and rotating in the reverse direction, the block member 42 is readily brought back to the threaded portion 44A for engagement therewith by the urging force of the plate spring 50A or 50B thus returning to the normal operation.
Another conventional lens driving mechanism is disclosed in Japanese Patent No. 3809026, in which an idle region for preventing forced engagement is provided at an output shaft of a driving source for a lens holder. Referring to FIG. 7, the abovementioned lens driving mechanism for an imaging device includes: a three-arm frame 51 adapted to hold a lens P and including a bearing portion to support a guide rod 56 for guiding the lens P in the direction of the optical axis of the lens P; a driving mechanism including a lead screw 53a provided with a threaded portion 53b to engage with a nut portion 52, and a stepping motor 53 to rotate the lead screw 53a; an arm 54 extending from the three-arm frame 51 so as to make contact with the nut portion 52 of the driving mechanism wherein the three-arm frame 51 is moved according to the movement of the nut portion 52; a spring 55 set in contact with an end of the three-arm frame 51 and adapted to work on the three-arm frame 51 so as to urge the arm 54 toward the nut portion 52; and an idle region 53c disposed only at an end of the lead screw 53a located closer to the arm 54 than to the nut portion 54 and having a cylinder shape with a diameter to prohibit engagement with the nut portion 52. In the above lens driving mechanism, the idle region 53c is disposed only at one end of the lead screw 53a, wherein the nut portion 52 is moved toward the idle region 53c by a control means when an error occurs.
Still another conventional lens driving mechanism (lens focus system) is disclosed in Japanese Patent Application Laid-Open No. 2002-287002, in which a lens unit 60 is prevented from being urged into forced engagement with the proximal end portion or the distal end portion of a rotary shaft 63 of a stepping motor 62 as shown in FIG. 8. In the above lens focus system, the lens unit 60 is engaged with a threaded portion 61a of the rotary shaft 63 thereby causing the lens unit 60 to move along the rotary shaft 63 when the stepping motor 62 rotates, wherein idle regions 64b and 64c which allow the rotary shaft 63 to rotate idle with respect to the lens unit 60 are disposed respectively at the proximal and distal end portions of the rotary shaft 63 as a lead screw. The lens focus system further includes urging members 65 and 66 which work on the lens unit 60 so that the lens unit 60 stays within the threaded portion 61a of the rotary shaft 63.
In the lens driving mechanism disclosed in Japanese Patent Application Laid-Open No. 2000-275497, which substantially includes the stepping motor, the feed screw connected to the output shaft of the stepping motor and the frame member having a square U shape and adapted to support the feed screw, since the plate springs as urging members must be provided at the frame member, the number of components is increased, the size is increased, and the cost of the entire mechanism cannot be reduced.
In the lens driving mechanism disclosed in Japanese Patent No. 3809026, the idle region is provided only at one end of the lead screw, and therefore the lens cylinder can be shortened. However, the nut portion must be moved toward the idle region for each initial alignment of the lens thus causing a delay corresponding to the move time and the movement is forced to start from the initially set position. Also, one end of the three-arm frame located opposite to the end in contact with the spring is caused to bump against another constituent member, and therefore the constituent members concerned here must have a high strength.
In the lens driving mechanism disclosed in Japanese Patent Application Laid-Open No. 2002-287002, both end portions of the rotary shaft are each provided with an idle region so as to sandwich the threaded portion, a guide rod is inserted through the lens unit which is disposed movably in a housing, and first and second springs as urging means are disposed respectively at both ends of the housing. As a result, the number of components is increased, the size is increased and the cost of the entire mechanism cannot be reduced like in the mechanism of Japanese Patent Application Laid-Open No. 2000-275497.